The present invention relates to low temperature processes for forming corrosion-inhibiting ceramic passivation coatings on ferrous substrates. In particular, the invention relates to forming passivation coatings at low temperatures using an aqueous basic metal hydroxide treatment bath containing SiO.sub.2 and a water-soluble glycol.
Coatings that provide a passivating barrier of exceedingly low solubility between a metal and its environment, through conversion of the metal surface into a corrosion-resistant, nonreactive form, play an important role in coating technology. Chemical conversion coatings are formed by a chemical oxidation-reduction reaction of the surface of a metal with a suitable chemical solution. This is in contrast to paints and most metallic coatings that require no chemical reaction with the base metal. Conversion coatings find wide-spread applications because they are particularly useful as primer coatings for paints, enamels and lacquers.
Other applications for conversion coatings depend on the natural color and protective value of the coating. Conversion coatings are often absorbent, providing an ideal base for protective oils, waxes or dyes. Conversion coatings are applied to iron and steel to provide a base for organic coatings, to aid in cold forming, to improve wear resistance, or to impart color and a degree of corrosion protection to the surface.
Conversion coatings can also be used as the protective coating of brake rotors and high-temperature broilers, and for other high-temperature applications. Corrosion-resistant coatings for brake rotors and boiler inner walls must also have properties such as hardness, abrasion resistance, adhesion and thermal stability. Chromate and phosphate conversion coatings have poor abrasion resistance and thermal stability. Even low temperature heating is deleterious to most chromate and phosphate coatings because protective qualities are lost with the loss of water. It has been observed that zinc phosphate coatings heated in the absence of air lose their corrosion resistance at between 150.degree. and 163.degree. C. In the case of chromate coatings, temperatures above 65.degree. C. in anhydrous environments should be avoided. Chromate and phosphate conversion coatings are also undesirable because the chemical agents used for their preparation include the highly toxic hydrazine, and the coating process pollutes the environment with chromate and phosphate ions.
Oxide coatings have good abrasion resistance and thermal stability. The process does not involve hydrogen embrittlement, so stressed parts can be treated. The small dimensional change resulting from the oxidation permits the treatment of precision parts.
Oxide coatings on ferrous substrates can be prepared by controlled high-temperature oxidation in air or by immersion in hot concentrated alkali solutions containing persulfates, nitrates or chlorates. Such coatings consist mostly of magnetite and do not protect against corrosion. Because oxide films are less porous than phosphate and chromate films, oxide films serve as a suitable base for oil, wax or paint coatings, with which some corrosion protection is obtained.
Surface conversion treatments include chemical conversion treatments obtained by dipping, spraying, brushing or swabbing without the use of external current, and anodic conversion obtained by processes in which the workpiece being treated functions as the anode in an electrolytic reaction. The coatings formed by these methods utilize phosphates, chromates, oxides, or combinations thereof, under carefully controlled conditions.
Most commonly, phosphate hydroxide coatings are formed on steel, which is referred to as Parkerizing or Bonderizing. The coatings are produced by brushing or spraying a cold or hot dilute manganese or zinc acid orthophosphate solution onto a clean surface of steel. This step removes the hydrogen developed on the surface of the coating so that the chemical reaction can occur to deposit complex iron and zinc phosphate crystals.
Iron phosphate is most conveniently applied to ferrous substrates, but zinc phosphate is more suitable as a primer coat. Phosphate coatings alone do not provide appreciable corrosion protection, but are useful mainly as a base for paints, ensuring good adherence and decreasing the tendency for corrosion to undercut the paint film at scratches or other defects. Phosphate coatings may also be impregnated with oils or waxes that provide a degree of protection against rusting, especially if corrosion inhibitors are also employed.
Chromate reactions are similar, utilizing chromium in the trivalent and hexavalent states. Chromate conversion coatings are produced on zinc by immersing the cleaned metal for a few seconds on sodium dichromate solution, acidified with sulftiric acid at room temperature, followed by rinsing and drying. A zinc chromate surface increases the life of zinc to a modest degree on exposure to the atmosphere. Despite the effectiveness of chromates in stopping the rusting of ferrous substrates in aqueous solutions, no successful chromate film process has been developed for this purpose. However, the corrosion resistance of a phosphate coating is enhanced by a dip or rinse in an acid chromate solution.
Acmite (NaFeSi.sub.2 O.sub.6) is a rock-forming mineral of the pyroxene group. It occurs primarily as a product of late crystallization of alkaline magmas. Acmite is very stable under hydrothermal conditions, even at high temperature and pressure, making it an ideal passivation layer candidate. Furthermore, the chemical agents used to prepare acmite coatings do not pollute the environment.
Mild steel is used to line the inner walls of quartz reactors because of the acmite passivation layers that form under the conditions typically employed in a quartz reactor. Bailey, Amer, J. Sci., 267a, 1-16, (1969), reports that acmite is stable over the temperature range of 550-850.degree. C. and pressure range of 20-500 MPa. Reaction kinetics therefore could be a factor influencing the minimum temperature to obtain a reaction product. In many cases, dissolution of an oxide is considered to be the rate-determining step for a hydrothermal reaction. See Eckert et al., J.Am.Ceram.Soc., 79(11) 2929-39 (1996)and Rossetti, et al., J. Crystal Growth, 116, 251-259 (1992). Laine et al., Nature, 353, 642-644 (1991) have shown that the use of glycols can dissolve otherwise poorly soluble oxides at temperatures as low as 198.degree. C. at atmospheric pressure.